Answer:
a. Columbian Exchange
Step-by-step explanation:
The first Europeans that managed to get tot he New World discovered lot of things that can be used back in their homelands. Lot of new crops were discovered, animals, vast fertile lands, as well as gold and silver. Once these things were introduced back in Europe the word quickly spread out, so the interest for exploration skyrocketed. Lot of people wanted to get out of Europe and go into the New World in order to get to this things, start a new life, and get rich. This resulted in countless explorations and mass migrations toward the New World.